1. Field of the Invention
This invention relates to the transmission and reception of information and particularly to the transmission and reception of information in digital form at frequencies which are hostile from the point of view of interference from other signals. More particularly, the invention is described in relation to transmitting in or close to the frequency bands of existing ultra-high frequency (UHF) TV signals.
2. Related Art
There exist channels in the UHF TV Spectrum which are not used as part of the frequency planning rules--these are known as the "taboo" channels. To understand these taboo channels it is necessary to have a brief understanding of the way in which the UHF TV band is planned.
The following description is given in the context of the frequency plan adopted in the United Kingdom. It will be understood by a person skilled in the art that, for similar reasons to those discussed below, taboo channels exist in the frequency plans of other countries and that the techniques described below for avoiding interference in a new low power service in a taboo channel may be applied in those countries also (with suitable alterations taking into account the different respective channel bandwidths/channel spacings and sub-carrier frequencies in those countries).
The frequency plan in the United Kingdom consists of 51 main transmitter stations covering some 90% of the population using horizontal polarisation. There then exists 950 small low power relay stations filling the main coverage gaps these use vertical polarisation. Each main transmitter station has a certain coverage area and needs perhaps 20 relay stations for gap filling.
The relay stations in a given coverage area of a single main station have restrictions on the frequencies to which they may be assigned because of the frequency planning taboos. Some of the taboos came about as the result of limited technology when the original UHF plan was designed back in 1961.
In the United Kingdom television channels are assigned 8 MHz segments of the frequency spectrum. If it is desired to broadcast television signals in channel number N, then a first pair of taboo channels (adjacent) arise at channel numbers N.+-.1 because, with receiver technology as it was in 1961, the receiver filters accepting channel N could not reject frequencies used by channels N.+-.1. Two other pairs of taboo channels (local oscillator and image channel) also arise at channels numbers N.+-.5 and N.+-.9 respectively because of the heterodyning process used to demodulate received television signals. If a first receiver were to receive a broadcast signal at one of channels numbers N.+-.5 then during the demodulation process frequencies would be generated at the receiver which would propagate and could interfere with operation of a nearby receiver attempting to demodulate a broadcast signal at channel number N.
There is now interest in exploiting these taboo channels in a way which does no cause interference to the existing television service. The present invention may be utilized for this purpose. More generally, the invention may be applied to enable the transmission of relatively low power signals in frequency bands subject to interference from other transmissions.
Transmissions in the taboo channels may take place without causing interference to existing relay stations in surrounding coverage areas which use the same frequencies providing very low power transmissions are used in the taboo channels. This criterion can be met by using digital modulation which enables transmitter power to be very much reduced without significantly reducing the coverage area. Typically, a digital signal may be transmitted using the methods of the present invention with 30 dB less power for approximately the same coverage as analogue amplitude modulation (AM). However, when sharing the UHF band at such low levels of transmitted power the digital signal is very vulnerable to interference from the much higher power levels of the existing services.
A proposal has been made in European patent application EP-A-0278192 to transmit digital data in the same channel as a conventional television signal. In this proposal the data to be transmitted is used to modulate the carriers of an orthogonal frequency division multiplex OFDM signal. Interference of the television signal into the OFDM signal is reduced by using a frequency offset technique. This technique relies on the fact that the energy in the frequency spectrum of a conventional television signal is centered around multiples of the line frequency 15625 Hz. The carriers of the OFDM signal are conditioned to exist only at frequencies which are offset from the line repetition "harmonic" frequencies of the existing television signal.
There is a finer repetitive structure to the conventional television signal spectrum arising because of the frame repetition rate 25 Hz. EP-A-0278192 also proposes a precision offset technique in which the carriers of the OFDM signal are conditioned to exist only at frequencies which are offset from these frame repetition "harmonic" frequencies.
Offset and precision offset techniques are well-known for use in reducing interference between broadcast television signals. For example, television transmitters broadcasting the same channel are arranged to broadcast their signals at frequencies offset from one another so that the line structure of one spectrum interleaves with that of the other. See EBU technical document 3254. However when contemplating applying an offset technique to an OFDM signal there is a difficulty.
When reference is made to an OFDM signal the image generally brought to mind is of a signal including orthogonal carriers overlapping by 50%, such as that having a power spectrum as illustrated in FIG. 1a. With such a signal the overall data transmission rate for the full channel bandwidth almost reaches the ideal Nyquist rate (see U.S. Pat. No. 3,488,445 in the name of Chang). It may be seen from FIG. 1b that such a signal containing overlapping carriers cannot be interleaved with a conventional television signal.
In order to implement an offset or precision offset technique using an OFDM signal it is proposed in EP-A-0278192 to dispense with overlapping OFDM carriers and instead to use a set of carriers spaced apart from one another and each having a narrower width of the carrier peak. Such an OFDM signal may be used in an offset or precision offset technique as illustrated by FIG. 1c.
The above system has the disadvantage that the overall data transmission rate of the OFDM signal is drastically reduced compared with the theoretical maximum. Furthermore, if a precision offset technique is used then the frequencies of the OFDM carriers must be very precisely locked to the carrier frequency of the interfering television signal.